57
its internal machinery. The energy absorbed by the chlorophyll molecules of the unstable autumn leaf is not
immediately channelled into useful products and processes, as it would be in an intact summer leaf. The
weakened fall leaf then becomes vulnerable to the highly destructive effects of the oxygen created by the
excited chlorophyll molecules.
H
Even if you had never suspected that this is what was going on when leaves turn red, there are clues out there.
One is straightforward: on many trees, the leaves that are the reddest are those on the side of the tree which
gets most sun. Not only that, but the red is brighter on the upper side of the leaf. It has also been recognised for
decades that the best conditions for intense red colours are dry, sunny days and coo nights, conditions that
nicely match those that make leaves susceptible to excess light. And finally, trees such as maples usually get
much redder the more north you travel in the northern hemisphere. It’s colder there, they’re more stressed,
their chlorophyll is more sensitive and it needs more sunblock.
I
What is still not fully understood, however, is why some trees resort to producing red pigments while others
don’t bother, and simply reveal their orange or yellow hues. Do these trees have other means at their disposal
to prevent overexposure to light in autumn? Their story, though not as spectacular to the eye, will surely turn
out to be as subtle and as complex.
